SigSize returns the needed size for signature buffer. So resize buffer to appropriate length and call it the second time.
0x6001 is the constant for SB_EC_FLD_TYPE_FP, so everything is ok with this (for unknown field it should be 0x6000).

Thank you for your help. I encoded succefully the signature but still verification fails..

I was confused with the result of ElECKeyMaterial.FieldType, because according to the help file 0x6001 is the constant for SB_EC_FLD_TYPE_UNKNOWN, and
0x6002 is the constant for SB_EC_FLD_TYPE_FP. There is no 0x6000 value...

Unfortunatelly I cannot provide the public key and ecc parameters as we have signed an NDA with our customer but I can ensure you that the ecc parameters and keys are valid.
The only thing I can provide helpdesk is a working test program I had wrote in c++ using openssl libraries and the non-working c# program I try to write with your library.
Do you think that this would be helpful?

Problem resolved with the valuable help of ELDOS's support team, after being moved to HelpDesk. Thank you for your prompt responses!

The main problem was in the negative value of 'a' parameter, which should be converted to positive integer by adding p, before converting to byte array.
The second problem was the missing of the signature encoding function.

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